The invention relates generally to sealed silos and more particularly to air breather bags which maintain pressure equilibrium between the inside and the outside of the sealed silo. The silo is recognized as a fundamental development in the art of storing silage and fodder materials which are utilized as feed for various farm animals. It has been determined that the storage life of various silage and fodder feed is extended markedly by preserving such feed in an atmosphere devoid of oxygen. Recently, therefore, silos have been designed to be substantially airtight to prevent the ingestion of oxygen laden air. Such a sealed silo is manufactured by A. O. Smith Harvestore Products, Inc., of Arlington Heights, Ill.
Such sealed silos may be forty to as much as sixty feet high and have diameters from twenty to thirty feet. Basically, therefore, the structures define sealed cylinders. Since they are located out of doors, they are subject to wide variations in temperature, produced by both the sun's radiant energy and ambient conditions, as well as variations in the barometric pressure. Absent a structure to compensate for the variations in internal pressure, the walls of a sealed silo would be subjected to substantial hoop stresses when the internal pressure was above the ambient and, likewise, subjected to crushing and collapsing forces when the reverse condition was true. It was therefore appreciated that a pressure equalizing device which maintained the silo in a sealed condition must be incorporated thereinto.
Such a pressure equalizing device is the air breather bag of the instant invention. Numerous designs have been suggested. Early breather device patents teach the use of long rectangular plastic breather bags or toroidal bags having a semi-circular profile. A configuration which has become somewhat standard within the industry comprises two separate arcuate, half-toroidal sections each having a circular cross section. Each half toroidal section is hung near the top of the silo with its ends generally abutting the ends of the other breather bag. The use of two half-toroid bags not only minimizes replacement costs resulting from the failure of a portion of one breather bag but also facilitates their manufacture and installation. Alternatively, three bags, each subtending an arc of approximately 120.degree., may be desirable to further reduce individual cost and further facilitate manufacture.
The present sophistication of silo breather bags is the result of various attempts to improve the service life of such breather bags through the strategic location of seams along lines of low stress and sealing methods which inhibit stress concentrations. An early breather bag seam configuration comprehends the use of a plurality of axially abutting bands. Such a design often utilized as many as twelve plastic panels which were formed into hoops and subsequently axially aligned and sealed. Finally, circular end panels were secured to opposite ends of the generally toroidal breather bag. The resulting breather bag exhibited a high seam length to volume ratio and was subject to early failure which invariably occurred along a seam.
Another previous design is disclosed in U.S. Pat. No. 3,888,288 to Hickle and Sherbourne which discloses a half-toroidal breather bag which includes staggered non-coextensive sealed edges which have no more than three sealed panels contiguous at any one point.